Pulsed fiber-MOPAs are increasingly being used in applications that require compact and robust monolithic design, good stability and excellent beam quality. Fiber-amplifiers exhibit much higher gain, typically between about 20 decibels (dB) and 40 dB, than solid-state amplifiers (typically between about 10 dB and 20 dB). This makes fiber-MOPAs attractive for amplification of a small signal (seed-pulse) from a master oscillator to high average and peak powers.
A master oscillator (for example, semiconductor diode-laser) can be easily modulated at a high pulse-repetition rate (PRF), for example, up to about 1 Gigahertz (GHz) while generating pulses with an arbitrary length, for example between about 0.1 nanoseconds (ns) and 10 microseconds (μs). This is one reason why fiber-MOPA systems offer better flexibility and choice in pulse-lengths (pulse-durations) and PRFs than solid-state lasers.
Most fiber-lasers operate at a wavelength in an infrared (IR) wavelength range. There is also, however, a growing demand for reliable, compact, pulsed laser-sources in visible and ultraviolet (UV) spectral ranges. This could be satisfied by frequency-converting the output of infrared-laser sources.
In general, pulses having a narrow bandwidth (less than about 0.6 nm), linear polarization, and high peak-power, for example greater than about 1 kilowatt (kW) are required for efficient conversion of IR radiation into visible and UV range. However, conventional high-power fiber-laser oscillators usually operate with broader bandwidths for example greater than about 1 nm. Further, high peak-power required for efficient harmonic generation is limited by nonlinear effects in fibers such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and Four-Wave Mixing (FWM). These effects further broaden pulse bandwidth.
A limited peak-power results in a limited average power at a fixed PRF and pulse-duration. One method for overcoming this peak-power limitation is described in U.S. Pat. No. 7,920,606. In this method seed-pulses, or pre-amplified seed-pulses, having a fundamental wavelength are split into first and second portions with the portions amplified separately in respectively first and second fiber-amplifiers. Amplified pulses from one of the fiber-amplifiers are frequency-doubled by second-harmonic generation (2HG) in one optically nonlinear crystal. These 2H-pulses are then sum-frequency mixed in another optically nonlinear crystal with amplified fundamental wavelength pulses from the other fiber-amplifier to generate frequency-tripled (third-harmonic) pulses. This method provides double the total power of a single amplifier for the third-harmonic generation (3HG).
A drawback of this method of course is that two fiber-amplifiers (power amplifiers) must be provided, each requiring pump-diodes and associated pump-light coupling optics. This adds significantly to the cost of the system. There is a need for fiber-MOPA system that overcomes the peak-power limitation of fiber-amplifiers with only one power fiber-amplifier or fiber-amplifier chain.